patch 8.2.0614: get ml_get error when deleting a line in 'completefunc'

Problem:    Get ml_get error when deleting a line in 'completefunc'. (Yegappan
            Lakshmanan)
Solution:   Lock the text while evaluating 'completefunc'.
diff --git a/runtime/doc/insert.txt b/runtime/doc/insert.txt
index 99c2d40..ff74d62 100644
--- a/runtime/doc/insert.txt
+++ b/runtime/doc/insert.txt
@@ -666,8 +666,10 @@
 ends CTRL-X mode (any key that is not a valid CTRL-X mode command) is mapped.
 Also, when doing completion with 'complete' mappings apply as usual.
 
-Note: While completion is active Insert mode can't be used recursively.
-Mappings that somehow invoke ":normal i.." will generate an E523 error.
+							*E565*
+Note: While completion is active Insert mode can't be used recursively and
+buffer text cannot be changed.  Mappings that somehow invoke ":normal i.."
+will generate an E565 error.
 
 The following mappings are suggested to make typing the completion commands
 a bit easier (although they will hide other commands): >